1. Field of the Invention
This invention relates to a focusing rear conversion lens versatilely usable with all photographic objective lenses.
2. Description of the Prior Art
As the methods of focusing a photographic objective lens, there are known various methods such as the method comprising moving the entire objective lens system, the method comprising moving only some of the lenses within or rearward of the lens system, etc. In these focusing methods, optimum focusing systems are adopted depending on the specification of the objective lens, namely, depending on whether the objective lens is telephoto, a wide angle of view or zoom or up to what degree of photographic magnification the objective lens can be focused to a short distance object.
This has prevented a common focusing device from being used versatilely and has been very inconvenient where an automatic focusing device is provided in a camera. A basic construction for a single lens reflex camera in which a lens system exclusively for focusing is mounted between the objective lens and the camera body to enable automatic focusing is known, for example, from Japanese Laid-open Patent Application No. 28133/1979. However, in the device disclosed in this patent application, a focusing converter is comprised of a movable negative lens group and an imaging positive lens group and the focus detection is effected by a parallel light beam system between the two lens groups, and this unavoidably leads to a considerably large shape as an attachment. In order to make the device mountable on all objective lenses, it is necessary to make the device into a compact one mountable to both an objective lens of great aperture ratio and an objective lens of short back focal length and moreover maintain an excellent imaging performance, and the designing of a converter which will satisfy all these conditions has been very difficult.